XC4000, XC4000A, XC4000H Logic Cell Array Families
pass through a global buffer before arriving at the IOB. This
eliminates the possibility of a data hold-time requirement
at the external pin. The I1 and I2 signals that exit the block
can each carry either the direct or registered input signal.
Output signals can be inverted or not inverted, and can
pass directly to the pad or be stored in an edge-triggered
flip-flop. Optionally, an output enable signal can be used to
place the output buffer in a high-impedance state, imple-
menting 3-state outputs or bidirectional I/O. Under con-
figuration control, the output (OUT) and output enable
(OE) signals can be inverted, and the slew rate of the
output buffer can be reduced to minimize power bus
transients when switching non-critical signals. Each
XC4000-families output buffer is capable of sinking 12 mA;
two adjacent output buffers can be wire-ANDed externally
to sink up to 24 mA. In the XC4000A and XC4000H
families, each output buffer can sink 24 mA.
There are a number of other programmable options in the
IOB. Programmable pull-up and pull-down resistors are
useful for tying unused pins to V
CC
or ground to minimize
power consumption. Separate clock signals are provided
for the input and output registers; these clocks can be
inverted, generating either falling-edge or rising-edge trig-
gered flip-flops. As is the case with the CLB registers, a
global set/reset signal can be used to set or clear the input
and output registers whenever the RESET net is active.
Embedded logic attached to the IOBs contains test struc-
tures compatible with IEEE Standard 1149.1 for boundary-
scan testing, permitting easy chip and board-level testing.
Programmable Interconnect
All internal connections are composed of metal segments
with programmable switching points to implement the
desired routing. An abundance of different routing re-
sources is provided to achieve efficient automated routing.
The number of routing channels is scaled to the size of the
array; i.e., it increases with array size.
In previous generations of LCAs, the logic-block inputs
were located on the top, left, and bottom of the block;
outputs exited the block on the right, favoring left-to-right
data flow through the device. For the third-generation
family, the CLB inputs and outputs are distributed on all
four sides of the block, providing additional routing flexibil-
ity (Figure 6). In general, the entire architecture is more
symmetrical and regular than that of earlier generations,
and is more suited to well-established placement and
routing algorithms developed for conventional mask- pro-
grammed gate-array design.
There are three main types of interconnect, distinguished
by the relative length of their segments: single-length lines,
double-length lines, and Longlines. Note: The number of
routing channels shown in Figures 6 and 9 are for illustra-
tion purposes only; the actual number of routing channels
varies with array size. The routing scheme was designed
for minimum resistance and capacitance of the average
routing path, resulting in significant performance improve-
ments.
The single-length lines are a grid of horizontal and vertical
lines that intersect at a Switch Matrix between each block.
Figure 6 illustrates the single-length interconnect lines
Slew Rate
Control
Passive
Pull-Up/
Pull-Down
Switch
Matrix
Switch
Matrix
OE
D
Out
Q
Output
Buffer
F4
G1
Pad
C4
G4
YQ
Y
G3
Flip-
Flop
C1
K
F1
CLB
Output
Clock
I
1
Input
Buffer
I
2
Q D
Flip-
Flop/
Latch
C3
F3
XQ
F2
C2
G2
X
Delay
Switch
Matrix
Switch
Matrix
Input
Clock
X6073
X3242
Figure 5. XC4000 and XC4000A Families
Input/Output Block
Figure 6. Typical CLB Connections to Adjacent
Single-Length Lines
2-14
XILINX [ XILINX, INC ]
